1. Field of the Invention
The present invention relates to a mode locking semiconductor laser system with an external cavity, and more particularly to a mode locking semiconductor laser system with an external cavity for emitting an ultra-short optical pulse for optical measurement techniques such as an (optical signal waveform measurement at an ultra high speed.
2. Description of the Related Art
Since the recent progress for optical technology enables the emission of the ultra-short optical pulse in femtosecond order, instead of making electrical measurements with such equipment as sampling oscilloscopes, it has been attempted that the ultra-short optical pulse be used as a sampling gate pulse for measuring an ultra high speed phenomenon.
The ultra-short optical pulse is used as the sampling gate pulse for measuring optical sampling waveforms, for example, an optical measurement for eye patterns of the optical waveform. For measuring individual sampling values, it is necessary that respective cross-correlation signal optical pulses are photoelectrically converted without any interference with any adjacent pulse. For this purpose, it is further necessary that a cyclic frequency of the cross-correlation signal optical pulse or a cyclic frequency of the sampling optical pulse is set under a frequency band of an optical receiving system. For this reason, differently from optical communication, a string of optical pulses with a low cyclic frequency of not more than 1GHz is needed.
For improvement in resolving power, the ultra-short optical pulse of a few picoseconds to a few femtoseconds is desirable. The mode locking semiconductor laser utilizing the external cavity is usable for emitting strings of the ultra-short optical pulses of the low cyclic frequency.
FIG. 1 is a schematic view of a conventional mode locking semiconductor laser system with the external cavity. The mode locking semiconductor laser system includes a semiconductor laser device 1 which includes a gain region 1a and a saturable absorbing region 1b with a saturable absorbing region facet 11b. The mode locking semiconductor laser system further includes a terminal reflective mirror 8 which forms an external cavity in cooperation with the saturable absorbing region facet 11b. Namely, the external cavity is defined between the terminal reflective mirror 8 and the saturable absorbing region facet 11b. 
The mode locking semiconductor laser system further includes a first collimator lens 2a, a wavelength splitter 3, and an intermediate reflective mirror 7 which are aligned on an optical axis between the semiconductor laser device 1 and the reflective mirror 8. The first collimator lens 2a is disposed between the semiconductor laser device 1 and the wavelength splitter 3. The wavelength splitter 3 is disposed between the first collimator lens 2a and the intermediate reflective mirror 7. The intermediate reflective mirror 7 is provided at an intermediate point on the optical axis between the wavelength splitter 3 and the terminal reflective mirror 8.
The mode locking semiconductor laser system further includes a second collimator lens 2b for allowing an optical output emitted from the saturable absorbing region facet 11b to be transmitted through the second collimator lens 2b. A wavelength of the laser beam emitted from the saturable absorbing region facet 11b depends on an angle of the wavelength splitter 3 with reference to the optical axis of the external cavity.
The intermediate reflective mirror 7 allows the size down of the mode locking semiconductor laser system, but would be an optical element for the mode locking semiconductor laser system. A cavity length is defined between the saturable absorbing region facet 11b and the reflective mirror 8. The intermediate reflective mirror 7 is movable in a direction labeled with an arrow mark xe2x80x9cAxe2x80x9d which is parallel to the optical axis. The movement or displacement of the intermediate reflective mirror 7 varies the cavity length. Variation of the cavity length varies the cyclic frequency or the cycle of reciprocal of the optical pulse in the cavity.
For optical measurement, it is important that the oscillation frequency, the cyclic frequency, the polarization state, and the optical output intensity are highly stable. For example, in order for detecting a variation of a plane of a polarized wave in accordance with a driving voltage for driving a circuit to be measured, the plane of the polarized wave is kept constant for a highly accurate measurement. For time sequential sampling, sampling pulses with a stable cyclic frequency are needed. For example, in case of a cyclic frequency of 1 GHz with a fluctuation of not more than several tens Hz order, a high stability of the frequency in the order of 1E-7 to 1E-8 order is necessary. It is further necessary that the cyclic frequency of the sampling pulses follows to the fluctuation of the cyclic frequency of the measured light or the measured signal.
The conventional mode locking semiconductor laser system of FIG. 1 allows that the mode locking frequency varies beyond an acceptable range from a target frequency due to a cavity length variation caused by a temperature variation.
The mode locking semiconductor device having a cyclic frequency (mode locking oscillation frequency) of not more than 1 GHz needs the external cavity with a cavity length of ten centimeters to several tens of centimeters. The mode locking semiconductor laser system of FIG. 1 utilizes a free space for the optical path, for which reason the cavity length is long. If the cyclic frequency is 1 GHz, the cavity length is 15 centimeters. If the cyclic frequency is 250 MHz, the cavity length is 60 centimeters. It is difficult that the long optical path is accommodated within a narrow space. It is difficult to reduce the size of the mode locking semiconductor laser system. The large size mode locking semiconductor laser system is disadvantageous in that a slight vibration or a slight strain may cause a relatively large displacement of parts and the mode locking semiconductor laser system is likely to receive influences of the temperature variation and the mechanical vibration. The large size mode locking semiconductor laser system is likely to allow that the oscillation frequency, the cyclic frequency, the polarization state and the optical output intensity vary beyond respective acceptable ranges. It is difficult to obtain a desirable long time and highly stable operation.
In addition to the long cavity length, the intermediate reflective mirror 7 makes it difficult and inconvenient to adjust the optical axis of the cavity. If the mode locking semiconductor laser with the external cavity is incorporated into a measuring system, then the measuring system has a large size.
In the above circumstances, the development of a novel mode locking semiconductor laser system including a semiconductor laser device and an external cavity free from the above problems is desirable.
Accordingly, it is an object of the present invention to provide a novel mode locking semiconductor laser system including a semiconductor laser device and an external cavity free from the above problems.
It is a further object of the present invention to provide a novel mode locking semiconductor laser system including a semiconductor laser device and an external cavity, wherein the mode locking semiconductor laser system has a reduced size.
It is a still further object of the present invention to provide a novel mode locking semiconductor laser system including a semiconductor laser device and an external cavity, wherein the mode locking semiconductor laser system is superior in stability for a long time in oscillation frequency, cyclic frequency and polarized wave plane against temperature variation and mechanical vibration.
It is yet a further object of the present invention to provide a novel mode locking semiconductor laser system including a semiconductor laser device and an external cavity, wherein the mode locking semiconductor laser system is variable in oscillation frequency and cyclic frequency.
The present invention provides an optical system including a cavity which comprises a semiconductor light-emitting device, and an optical fiber having a first terminal optically coupled to the semiconductor light-emitting device, the cavity having a cavity length defined between a first facet of the semiconductor light-emitting device and a second terminal of the optical fiber, wherein a length of the optical fiber is such that a mode-locking oscillation frequency is not more than 1 GHz.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.